A variety of physical conditions involve two tissue surfaces that, for diagnosis or treatment of the condition, need to be separated or distracted or maintained in a separated condition from one another and then supported in a spaced-apart relationship. Such separation or distraction may be to gain exposure to selected tissue structures, to apply a therapeutic pressure to selected tissues, to return or reposition tissue structures to a more normal or original anatomic position and form, to deliver a drug or growth factor, to alter, influence or deter further growth of select tissues or to carry out other diagnostic or therapeutic procedures. Depending on the condition being treated, the tissue surfaces may be opposed or contiguous and may be bone, skin, soft tissue, or a combination thereof.
One such a condition that occurs in the orthopedic field is vertebral compression fractures. Vertebral compression fractures affect a significant part of the population, and add significant cost to the health care system. A vertebral compression fracture is a crushing or collapsing injury to one or more vertebrae. Vertebral fractures are generally but not exclusively associated with osteoporosis, metastasis, and/or trauma. Osteoporosis reduces bone density, thereby weakening bones and predisposing them to fracture. The osteoporosis-weakened vertebrae can collapse during normal activity and are also more vulnerable to injury from shock or other forces acting on the spine. In severe cases of osteoporosis, actions as simple as bending forward can be enough to cause a vertebral compression fracture. Vertebral compression fractures are the most common type of osteoporotic fractures according to the National Institute of Health.
The mechanism of such vertebral fractures is typically one of flexion with axial compression where even minor events can cause damage to the weakened bone. While the fractures may heal without intervention, the crushed bone may fail to heal adequately. Moreover, if the bones are allowed to heal on their own, the spine may be deformed to the extent the vertebrae were compressed by the fracture. Spinal deformity may lead to breathing and gastrointestinal complications, and adverse loading of adjacent vertebrae.
One technique used to treat vertebral compression fractures is injection of bone filler into the fractured vertebral body. This procedure is commonly referred to as percutaneous vertebroplasty. Vertebroplasty involves injecting bone filler (for example, bone cement, allograph material or autograph material) into the collapsed vertebra to stabilize and strengthen the crushed bone.
In vertebroplasty, physicians typically use one of two surgical approaches to access thoracic and lumbar vertebral bodies: transpedicular or extrapedicular. The transpedicular approach involves the placement of a needle or wire through the pedicle into the vertebral body, and the physician may choose to use either a unilateral access or bilateral transpedicular approach. The extrapedicular technique involves an entry point through the posterolateral corner of the vertebral body.
Regardless of the surgical approach, the physician generally places a small diameter guide wire or needle along the path intended for the bone filler delivery needle. The guide wire is advanced into the vertebral body under fluoroscopic guidance to the delivery point within the vertebra. The access channel into the vertebra may be enlarged to accommodate the delivery tube. In some cases, the delivery tube is placed directly into the vertebral body and forms its own opening. In other cases, an access cannula is placed over the guide wire and advanced into the vertebral body. After placement, the cannula is replaced with the delivery tube, which is passed over the guide wire or pin. In both cases, a hollow needle or similar tube is placed through the delivery tube into the vertebral body and used to deliver the bone filler into the vertebra.
In this procedure, the use of lower viscosity bone filler and higher injection pressures tend to disperse the bone filler throughout the vertebral body. However, such procedures dramatically increase the risk of bone filler extravasation from the vertebral body. The difficulty of controlling or stopping bone filler flow into injury-sensitive areas increases as the required pressure increases. Thus, caution must still be taken to prevent extravasation with the greatest attention given to preventing posterior extravasation because it may cause spinal cord trauma. Physicians typically use repeated fluoroscopic imaging to monitor bone filler propagation and to avoid flow into areas of critical concern. If a foraminal leak results, the patient may require surgical decompression and/or suffer paralysis.
Another type of treatment for vertebral fractures is known as Kyphoplasty. Kyphoplasty is a modified vertebral fracture treatment that uses one or two balloons, similar to angioplasty balloons, to attempt to reduce the fracture and, perhaps, restore some vertebral height prior to injecting the bone filler. One or two balloons are typically introduced into the vertebra via bilateral transpedicular cannula. The balloons are inflated to reduce the fracture. After the balloon(s) are deflated and removed, leaving a relatively empty cavity, bone cement is injected into the vertebra. In theory, inflation of the balloons may restore some vertebral height. However, in practice it is difficult to consistently attain meaningful and predictable height restoration. The inconsistent results may be due, in part, to the manner in which the balloon expands in a compressible media, such as the cancellous tissue within the vertebrae, and the structural orientation of the trabecular bone within the vertebra, although there may be additional factors as well.
Thus there is a need for devices and methods to treat the above mentioned diseases, in particular compression vertebral fractures.
Another location of the body where tissue separation is useful as a corrective treatment is in the spinal column. Developmental irregularities, trauma, tumors, stress and degenerative wear can cause defects in the spinal column for which surgical intervention is necessary. Some of the more common defects of the spinal column include vertebral compression fractures, degeneration or disruption of an intervertebral disk and intervertebral disk herniation. These and other pathologies of the spine are often treated with implants that can restore vertebral column height, immobilize or fuse adjacent vertebral bones, or function to provide flexibility and restore natural movement of the spinal column. Accordingly, different defects in the spinal column require different types of treatment, and the location and anatomy of the spine that requires corrective surgical procedures determines whether an immobilizing implantable device or a flexible implantable device is used for such treatment.
In a typical spinal corrective procedure involving distraction of tissue layers, damaged spinal tissue is removed or relocated prior to distraction. After the damaged tissue has been removed or relocated, adjacent spinal tissue layers, such as adjacent bone structures, are then distracted to separate and restore the proper distance between the adjacent tissue layers. Once the tissue layers have been separated by the proper distance, an immobilizing or flexible device, depending on the desired treatment, is implanted between the tissue layers. In the past, the implantable treatment devices have been relatively large cage-like devices that require invasive surgical techniques which require relative large incisions into the human spine. Such invasive surgical techniques often disrupt and disturb tissue surrounding the surgical site to the detriment of the patient.
Therefore, there remains a need for implantable treatment devices and methods that utilize minimally invasive procedures.
Such methods and devices may be particularly needed in the area of intervertebral or disk treatment. The intervertebral disk is divided into two distinct regions: the nucleus pulposus and the annulus fibrosus. The nucleus lies at the center of the disk and is surrounded and contained by the annulus. The annulus contains collagen fibers that form concentric lamellae that surround the nucleus and insert into the endplates of the adjacent vertebral bodies to form a reinforced structure. Cartilaginous endplates are located at the interface between the disk and the adjacent vertebral bodies.
The intervertebral disk is the largest avascular structure in the body. The cells of the disk receive nutrients and expel waste by diffusion through the adjacent vascularized endplates. The hygroscopic nature of the proteoglycan matrix secreted by cells of the nucleus operates to generate high intra-nuclear pressure. As the water content in the disk increases, the intra-nuclear pressure increases and the nucleus swells to increase the height of the disk. This swelling places the fibers of the annulus in tension. A normal disk has a height of about 10-15 mm.
There are many causes of disruption or degeneration of the intervertebral disk that can be generally categorized as mechanical, genetic and biochemical. Mechanical damage includes herniation in which a portion of the nucleus pulposus projects through a fissure or tear in the annulus fibrosus. Genetic and biochemical causes can result in changes in the extracellular matrix pattern of the disk and a decrease in biosynthesis of extracellular matrix components by the cells of the disk. Degeneration is a progressive process that usually begins with a decrease in the ability of the extracellular matrix in the central nucleus pulposus to bind water due to reduced proteoglycan content. With a loss of water content, the nucleus becomes desiccated resulting in a decrease in internal disk hydraulic pressure, and ultimately to a loss of disk height. This loss of disk height can cause the annulus to buckle with non-tensile loading and the annular lamellae to delaminate, resulting in annular fissures. Herniation may then occur as rupture leads to protrusion of the nucleus.
Proper disk height is necessary to ensure proper functionality of the intervertebral disk and spinal column. The disk serves several functions, although its primary function is to facilitate mobility of the spine. In addition, the disk provides for load bearing, load transfer and shock absorption between vertebral levels. The weight of the person generates a compressive load on the disks, but this load is not uniform during typical bending movements. During forward flexion, the posterior annular fibers are stretched while the anterior fibers are compressed. In addition, a translocation of the nucleus occurs as the center of gravity of the nucleus shifts away from the center and towards the extended side.
Changes in disk height can have both local and global effects. On the local (or cellular, level) decreased disk height results in increased pressure in the nucleus, which can lead to a decrease in cell matrix synthesis and an increase in cell necrosis and apoptosis. In addition, increases in intra-discal pressure create an unfavorable environment for fluid transfer into the disk, which can cause a further decrease in disk height.
Decreased disk height also results in significant changes in the global mechanical stability of the spine. With decreasing height of the disk, the facet joints bear increasing loads and may undergo hypertrophy and degeneration, and may even act as a source of pain over time. Increased stiffness of the spinal column and increased range of motion resulting from loss of disk height can lead to further instability of the spine, as well as back pain.
Radicular pain may result from a decrease in foraminal volume caused by decreased disk height. Specifically, as disk height decreases, the volume of the foraminal canal, through which the spinal nerve roots pass, decreases. This decrease may lead to spinal nerve impingement, with associated radiating pain and dysfunction
Finally, adjacent segment loading increases as the disk height decreases at a given level. The disks that must bear additional loading are now susceptible to accelerated degeneration and compromise, which may eventually propagate along the destabilized spinal column.
In spite of all of these detriments that accompany decreases in disk height, where the change in disk height is gradual many of the ill effects may be “tolerable” to the spine and patient and may allow time for the spinal system to adapt to the gradual changes. However, the sudden decrease in disk volume caused by herniation which requires surgical removal of the disk or disk nucleus may increase the local and global problems noted above.
Many disk defects are treated through a surgical procedure, such as a discectomy in which the nucleus pulposus material is removed. During a total discectomy, a substantial amount (and usually all) of the volume of the nucleus pulposus is removed and immediate loss of disk height and volume can result. Even with a partial discectomy, loss of disk height can ensue. Discectomy alone is the most common spinal surgical treatment, frequently used to treat radicular pain resulting from nerve impingement by disk bulge or disk fragments contacting the spinal neural structures.
The discectomy may be followed by an implant procedure in which a prosthesis is introduced into the cavity left in the disk space when the nucleus material is removed. Thus far, the most common prosthesis is a mechanical device or a “cage” that is sized to restore the proper disk height and is configured for fixation between adjacent vertebrae. These mechanical solutions take on a variety of forms, including solid kidney-shaped implants, hollow blocks filled with bone growth material, push-in implants and threaded cylindrical cages.
A challenge in the use of a posterior procedure to install spinal prosthesis devices is that a device large enough to contact the end plates and expand the space between the end plates of the same or adjacent vertebra must be inserted through a limited space. In the case of procedures to increasing intervertebral spacing, the difficulties are further increased by the presence of posterior osteophytes, which may cause “fish mouthing” or concavity of the posterior end plates and result in very limited access to the disk. A further challenge in degenerative disk spaces is the tendency of the disk space to assume a lenticular shape, which requires a relatively larger implant than often is difficult to introduce without causing trauma to the nerve roots. The size of rigid devices that may safely be introduced into the disk space is thereby limited.
While cages of the prior art have been generally successful in promoting fusion and approximating proper disk height, typically these cages have been inserted from the posterior approach, and are therefore limited in size by the interval between the nerve roots. Further, it is generally difficult, if not impossible to implant from the posterior approach a cage that accounts for the natural lordotic curve of the lumber spine.
It is desirable to reduce potential trauma to the nerve roots and yet still allow restoration or maintenance of disk space height in procedures involving vertebrae fusion devices and disk replacement, containment of the nucleus of the disk or prevention of herniation of the nucleus of the disk. In general minimally invasive surgical techniques reduce surgical trauma, blood loss and pain. However, despite the use of minimally invasive techniques, the implantation of cage devices for treating the spine typically involves nerve root retraction, an inherently high risk procedure. It is therefore desirable to reduce the degree of invasiveness of the surgical procedures required to implant the device, which may also serve to permit reduction in the pain, trauma, and blood loss as well as the avoidance and/or reduction of the nerve root retraction.
In minimally invasive procedures, to monitor placement, it is useful that implant devices inserted into spinal tissue be detectable using fluoroscopic imaging systems. However if a device is visible using X-ray technology, then the device can interfere with the detection and monitoring of spinal tissues, such as bone growing into the disk space after a vertebral fusion procedure. Additional advances would also be useful in this area.